The present invention generally relates to servicing of requests from computer peripherals. More particularly, the invention is directed to systems, methods and programs for managing the mode of operation used to service the requests of peripherals connected to a workstation processor.
Contemporary personal computers and workstations have processor and operating system resources to multi-task, have tremendous computational capability, and have input/output (I/O) resources to interface a diverse range of peripheral devices. For example, it is common to have a workstation I/O bus servicing a network communication adapter card, a printer interface, a keyboard interface, a mouse interface, and multiple non-volatile disk and tape media devices. The servicing of the devices by the workstation, whether it be by the central processor or an I/O processor, is conventionally initiated in one of two manners. The first practice involves the use of polling, whereby the processor successively queries the peripheral devices regarding their needs for servicing. The other approach involves the use of interrupts, where the peripheral devices signal the processor of a need for service. Each technique has its benefits and deficiencies.
In general, polling is considered to have a lower latency, in that the frequency of the queries to the peripheral devices can be made relatively fast. However, if the service needs of the peripheral devices are relatively low in occurrence, and the polling rate is fast to minimize latency, the processor performing the polling expends a disproportionately large amount of time polling in relation to other functions that the processor accomplishes.
An interrupt mode of operation, on the other hand, commonly exhibits a greater latency. This is attributable to the fact that the interrupt signal initiates an operating system routine, which software routine must first determine the origin of the interrupt and then evaluate the priority in relation to concurrent interrupt requests or an ongoing servicing. The disablement of an ongoing peripheral service as a consequence of a higher priority interrupt, and inherent delays associated with the determination of a priority for each interrupt, result in an average high latency for a high service rate interrupt mode I/O system. On the other hand, when the service demanded by the peripherals is relatively infrequent, the interrupt mode is on an average more processor efficient than the practice of polling.
The selection between a polling mode and an interrupt mode of servicing peripheral devices is integrated into the design of the I/O systems on the basis of anticipated usage. Unfortunately, the selection, irrespective of which mode is chosen, becomes unsuitable when the I/O system needs exhibit a mix of service request characteristics. For example, when the peripheral device service requests exhibit bursts of activity followed by extended periods of little or no activity, neither mode is particularly efficient.